The benefits of floppy props

The standard T-motor Air Gear 200 props are plastic and very flexible.  During takeoff they bend upwards as they spin up until the bend is strong enough to ‘lift’ the quad off the ground – this makes the lift very gentle.

In contrast, the CF props have no flex, any lift is directly and immediately applied to the frame, and therefore to the sensors.  The sensors feel everything; there’s no smoothing or caressing; sharp spikes in motor speed make it to the sensors, to the extent that acceleration may exceed the range of the sensors; I believe this is the cause of my negative G problem.

With the IMU configured to suppport a range of ±2g (the highest resolution), then an additional 1g spike a take-off will overflow.  With the hard light higher pitch CF props, it seems entirely possible this could happen.

There are several ways to fix this each with various pros and cons.  I could

  • increase the low pass filter, but this does still allow the overshoot but then filters it out, along with other valid data – this works though it’s a hack / workaround / sticky plaster, not a fix.
  • extend the range of the sensor to ±4g but there’s a corresponding reduction in sensor resolution which means larger undetectable drift
  • add physical buffering like the props do – I did this previously with the HoG floating on 8 very solt silicone gromits.  I don’t want to use this again (expensive and breakable) but something similar with thicker, softer foam tape sticking HoG to the frame could work
  • soften the flight plan transition in software.

What I’d going to do it a combination of safety precautions and diagnostics

  • I’ll add diagnostics to flag these 0g events, and to skip any data including such an event for safety reasons.
  • I’ll up the range of the sensors to ±4g, and add further diagnotics to flag overshoots over ±2g just to make sure I’m right!

I’ll decide the final solution based on what I find out.

 

Redecorating the lounge ceiling

I don’t like the props Zoe uses that come with the T-motor Air Gear 200 kit; they’re flimsy plastic, they bend and the stay bent.

A quick ebay browse over the weekend turned up some 3 blade carbon props, with the required 5mm bore hole, and 6″ span compared to the plastics’ 6.5″.  Perfect.  They arrived in the post this morning.

So I gave them a quick try prior to packing up Zoe for the work engineering conference tomorrow; the result?  I’m redecorating the lounge ceiling.

Swapping back to the flippy floppy props and normal service was resumed. Phew!

I’d seen this in the past with Phoebe and that triggered the new custom PCB for her, after which she no longer had the I2C errors, and I was able to swap her to the 1kHz sampling FIFO code.  Yet while testing this outdoors yesterday, she still lept into the air until I turned the alpf filter down from 0 (460Hz) down to at least 2 (96Hz).

The fact Zoe now does this too with the CF props is scary and fascinating.  Once I get back from the conference, I’ll be switching her over to the CF props and flying her outside with diagnostics enable to try to track down why such a simple swap of props yields such radically different behavior!

But for now, I need to get back to repainting the lounge ceiling before the wife and kids get home and spot the distruction!

Report on the Zoe at the Cotswold Jam

Zoe did her stuff at the Cotswold Jam reasonably well, but she consistently drifted to starboard (right), even when I was safety testing her outside before hand at a much lower temperature.

The previous day, I’d also safety tested her indoors and she drift consistently backwards are about the same speed.

The setup and software for the two days was the same, except I’d removed the props for transit to the jam.

Could the difference in drift direction simply be the props?   Had I rebuilt her at the jam with one diagonal pair of the props swapped compared to previously?  That could account for the 90° change of drift.  Certainly the change in drift does point heavily at the props.  The props are plastic and they bend easily and permanently.  So I now have some sturdy CF props on the way which I hope will limit the drift in time for my work engineering conference at the end of the week.

Angle of attack math(s) – not

I spent yesterday trying to work out why Chloe doesn’t fly well with the OEM T-series 1355 (13″ tip-to-tip, 0.55″ elevation) compared to both the T-Motor 1344 and Phoebe with her OEM 1155 props.

T-motor vs OEM AoA

T-motor vs OEM AoA

I did a whole load of experimentation, measuring the rotation frequency at hover of Phoebe (155Hz) and Chloe (135Hz) using the audio spectrum analyser app on my iPad, used SonicPi to confirm the frequencies matched what I heard (49 and 51 in MIDI numbers if you want to try) and started writing a long blog, but I couldn’t find anything that explained Chloe’s poor flights.

Then after a night’s sleep it became obvious.  Before every flight, there’s a second spent winding the props up to hover speed.  But since relying on the IMU interrupts to define time, somehow time has speeded up – 7s of flight according to the IMU sampling rate actually takes about 6.5s according to HoG’s python code.  And that means Chloe isn’t getting as much time to wind up her props to hover speed.  Upping that to 1.5s and all was behaving perfectly with the OEM props.

I’m not quite sure why the timing shifted when I moved to 500Hz sampling rate on the IMU – I’ll have a look but to be honest, I don’t care too much as everything is working well.

Angle of attack

I tried Chloe with the code changes I’d made for Phoebe, and she just wouldn’t behave; regardless of what PID tuning I did, she struggled to get herself off the ground, and once in the air, she wobbled enormously. Just like Phoebe, Chloe was flying with the super-cheap, super-strong OEM (other equipment manufacturer) versions of the T-motor props.

Finally I’ve remembered the correct term for the problem – the “angle of attack” of the props.  According to wikipedia, the critical or stalling angle of attack for a typical aerofoil is around 15° – 20° with respect to the airflow.  This is the point where the airflow at the leading edge of a prop starts to break off, producing turbulence over the tops side of the prop killing the lift.  To make things worse, the turbulence spreads and prevents good air-flow even over parts of the props whose angle of attack is under the critical angle.

With an aeroplane in flight, there is airflow at the plane’s flight speed through the prop – the pitch of the prop can be very high especially near the shaft as a result, and yet still the angle of attach stays under the critical limit..

But with a Quadcopter in hover, there is no airflow other than that created by the props themselves; hence the props angle-of-attack needs to be under the critical level when stationary.  Some simplistic measurements with a digital protractor suggests the OEM props pitch is about 28° at the steepest point, whereas that of the T-motors is about 17°.

So I popped some T-motor props back on Chloe to see what effect that had.  And Chloe behaved a lot closer to her previous behaviour.

But that raised the question of why Phoebe is flying so well with these OEM props?  Phoebe’s props spin a lot faster, so perhaps the airflow is high enough to keep the 28° pitch of the OEM props under the critical angle-of-attack?  Plausible, but just guesswork.

I’ll need to do more thinking, as I really want Chloe to be flying as well as Phoebe does with the OEM props – the price difference between these and the official T-motor equivalents is huge (a factor of 8).  I think the next step is to measure the rotation rate of Phoebe’s and Chloe’s motors in flight, and try to calculate the angle of attack for both.  I have an audio analyser app on my iPad which should identify the rotation rate as a peak in the FFT display.

This is one of those moments I wish I’d done aeronautical engineering at Southampton University rather than Physics at Cambridge, but I do have a book about Fluid Dynamics so I’ll take a look to see if there’s anything there.

Why is Phoebe so manic?

Given that Phoebe and Chloe are so similar, why is their behaviour so different?  Chloe is like a Mum: calm, mellow and thoughtful; Phoebe is like her 3 year old daughter, bouncing around, screaming with delight right to the point she hurts herself and starts crying*.

I’ve been thinking about what could be through cause of this behaviour; the HoG’s are identical, which means hardware.

Motors:

  • Chloe’s has T-motor MN3501-16 12N14P (12 coils, 14 magnets)
  • Phoebe’s has T-motors MT2216-11 12N14P (12 coils, 14 magnets)

Because they have the same coil / magnet configuration, if they have the same ESC with the same PWM pulse width feeding it, they will rotate at the same speed, so this isn’t it.

Props

  • Chloe’s has T-motor 13 x 4.4 CF props – the larger of the two sizes recommended for use with 11.1 batteries and the motors (above) she has
  • Phoebe’s has T-motor 11 x 3.7 CF props – the middle of the 3 sets of props recommended for use with 11.1V batteries, and the motors (above) she has

So assuming they use identical PWM and ESCs and the motors above with identical coil / magnets, how much power to their props generate?  Dunno, but given Chloe weighs more than Phoebe, and has longer arms than Phoebe, there’s some logic that suggests she needs proportionally larger bigger props, which she does.  So this doesn’t seem a likely cause.

ESCs

  • Chloe’s has T-motor 30A opto ESCs
  • Phoebe has DJI 30A opto ESCs

The translation of the PWM pulse to the motors coil switching is very specific to each ESC – the details can only be found from the code running on the microcontroller inside the ESC so I have little to really prove it’s the ESCs fault except DJI have replaced the ESCs I use with new ones – perhaps related to Phoebe’s nutty behaviour – at least that’s plausible.

And last, but not least comes Noise

  • Chloe’s HoG is attached to the rest of her frame with very soft silicone grommets; there is no hard physically connect between her HoG and the motor / prop noise transmitted through the rest of the frame.
  • Phoebe on the other hand is directly connected to the rest of the frame and feels everything.

I already have silicone grommets on order (and they should have arrived today), so I shall be fitting those as soon as they arrive.

I may well also buy new ESCs for Phoebe that match Chloe, but I’ll need to check my bank account first as they’ll be £125 for 4!

Why bother?  Because Phoebe is running through props at probably 10 times the rate Chloe does, and so even a change in ESCs will pay for itself within a month!

P.S. While I was search for what 12N14P meant, I found this article about how to wind the coils of a 12N14P motor – worth a read if only for background knowledge.


*Through experience within earshot right know, the comparison I’ve made between Phoebe and Chloe, and my wife and daughter is absolutely accurate!

Prop aerodynamics picture

Here’s a pics of  the two types of props I was describing yesterday.

Aerodynamic comparison

Aerodynamic comparison

Near the axel of the shiny cheap CF props, the prop surface is at 45° to the airflow, whereas with the T-motor props, it’s pretty much 0°.  The 45° props are ploughing through the air and as a result, it’s much more likely that the airflow breaks up into turbulence over the prop and lift is lost.  What’s more, once there, turbulence will spread along towards the tip of the prop, so a significant amount of lift is lost.  In comparison, the 0° props are slicing through the air and gently enticing it to flow smoothly across the prop aerofoil; risk of turbulence is much reduced, and so the props provide more lift.

P.S. I ought to explain my definition of ‘strong’ blades:  during tuning of the rotation PIDs, it’s not unusual for the quad to hit the ground on its side, with one prop embedded deep into the lawn.  In that situation, the cheap CF prop can hold the whole 1.5kg body of the quad up in the air whereas the T-motor ones cannot – they snap.  But given that’s way outside the ‘normal operation’ bounds of the prop design, it’s unfair to call the T-motors ‘weak’, they are more than strong enough for their intended purpose.  But the cheap CF props do keep the costs down when lawn-mowing is a routine part of flight testing!

Propeller aerodynamics 101

I know nigh on nothing about propeller aerodynamics; what I do know is based on common sense and evidence from today’s flights.

If you look at an aeroplane propeller, there’s quite a steep pitch angle near the axis of rotation.  In comparison, an aeroplane’s wings leading edge is virtually horizontal (zero pitch angle), and instead the wing ‘droops’ down towards the trailing edge.  That’s because the aeroplane’s propeller is effectively cork-screwing into a high speed airflow to provide propulsion along the same axis as the air-flow. In comparison, the wings are slicing through the same airflow to provide lift orthogonal to the airflow over them.

With a quadcopter (or helicopter), there is no similar cork-screw air-flow.  Event vertical movement is a slow airflow due to gravity.  And that means the props for a quadcopter need to slice rather than corkscrew through the air.

The expensive and breakable T-motors props are shaped like aeroplane wings.  The cheap CF props I swapped to because of their cost and strength are shaped like aeroplane propellers.

So when today on a whim, I swapped the cheap CF props with almost equivalently sized T-motor props, I had to do a huge amount of PID detuning; even though the T-motors’ were slightly smaller, their aerodynamics were like a ‘plane’s wing rather than a ‘plane’s propeller, and as a result the T-motor props produced a lot more power a lot more efficiently.

Luckily the cheap CF props have served their purpose well, getting HoG flying stably at minimal expense (£12 for 4).  But now I think I need to go back to the expensive (£80 for 4), but aerodynamically correct T-motor props for continued testing.  I just hope HoG doesn’t crash breaking a prop – I can’t afford £80 a time for replacements – that’s how I got into the financial mess I’m in at the moment breaking a set of these T-motors props at least once a week!

Odds ‘n’ Sods, Bits ‘n’ Bobs

  1. I got my Raspberry Pi 2 yesterday and set it up to replace my general workhorse – essentially my PC connected to the home network, and used to connect with all my other robotic / IoT projects.Completely as expected, the main change is in CPU usage – every time I started an app on the B+ the CPU went to 100% and stayed there for a few seconds; with the 2, it never rose above 30%.

    There was nothing really earth shattering in the performance of each app – I suspect because the userland code hasn’t been changed, but I suspect that multiple apps would run faster together due to the quad-core allowing them to run independently.

    After redistribution of RAM to 256MB to the GPU, leaving 768MB to the Quad CPU’s and over-clocking to 1GHz, the transformation is very noticeable: applications start instantly, and CPU usage never goes beyond 30%, whereas previous, apps could take several seconds to load, and CPU would be up at 100%.  So even as just a workhorse it’s now become a lot more usable as a desktop PC.  If I’ve time to kill, I might get LibreOffice and some games to see whether they perform to desktop PC requirements.

  2. I’ve got a large delivery of new props due today, but photographing the graveyard yesterday made me think twice about immediate deployment.  Instead, I’ve dug out my stock of cheap carbon blade, and will see how they perform instead while Zoë is still hitting walls.
  3. That change to use the old props was also partly triggered by a video on Vimeo from last summer, which was so much better than yesterday’s supposed “best flight ever”.  So I’m somewhat deflated; perhaps it’s just the weather – is Zoe SAD?The changes since that previous video include double of performance, temperature control, threading and gravity filtering.  Using the old props liberates me to tweak configuration and code, and not care too much about resultant impacts with the walls.

You put your right prop on..

your right prop off.
on, off, on, off
and smash into a wall.
You do the hokey cokey and you turn around.
That’s what it’s all about!

Why do all my crashes break a right hand, ACW / CCW rotating prop?  I have so many spare lefts it’s getting embarrassing!

Though it wouldn’t help that sometime (I think yesterday) I fitted 10″ instead of my normal 11″ props to Phoebe’s left hand side.  Oopsy – it least that explains the increasing negative (ACW) rotation, eventually leading to the left wall collision.

On the plus side, with the correct props fitted on the left hand side, I got stability matching the take-off angle with the attitude only PIDs (i.e. completely as expected), and zero-drift once the motion PIDs were included too (again, completely as expected).  In other words, absolutely perfect…

…except for the fact she kept climbing; nevertheless still worth videoing.  Two reasons why I didn’t video the flights – they’re called Jacob and Milly; I’m the parent in charge until mid-afternoon so my time for test flights today is very limited!

So back to the Z-axis velocity PIDs, it seems.  I wonder whether the dlpf of 3 (44Hz, 4.9ms lag) could be the cause – either too high or too low – I can speculate on the cause of either way, so I’ll try both dlpf 4 (20Hz, 8.3ms lag) and dlpf 2 (90Hz and 3.0ms lag).

Laters…